Pigments, resins, and solvents are major components of pigmented paints and inks. While resins are mainly responsible for the film formation and adhesion between coatings and substrates, pigments are widely used in paint/ink industries to impart aesthetic effects or protective functions to the coated substrates. The properties of pigmented paint/inks and the performances of the final coating are greatly affected by the dispersion of the pigments and their interaction with resin, solvent, and other components of the paints/inks. However, due to the complicated interactions among these ingredients, proper dispersion of pigments in their vehicles and optimal characteristics of the paints/inks are desirable.
Incorporation of pigments into inks and coatings is typically achieved by dispersing dry pigments into liquid resin systems. In order to achieve optimum hiding power or color strength, extensive shear has to be applied during the mixing process to break the pigment agglomerates or aggregates down into primary particles. The pigment particles also need to be stable in the paint/ink system during the storage as re-aggregation of the pigment particles or separation of pigment from the bulk of paint/ink system will reduce the performance of the paint/ink system.
The addition of pigment to the paint/ink system often has complicated implication to the performance of paint/ink systems. For example, the interaction between pigment surfaces and resin polymer chains can alter the viscosity profiles of the ink/paint systems. Depending on their application conditions, paints or inks have their distinctive preferred rheology profiles. The viscosity of the paint/ink products depends greatly not only on the size, shape, concentration, and dispersion of pigment particles, but also on the interactions between pigment particles and other components of the resin systems. On the other hand, incompatibility between resin and pigments often results in flocculation and separation of pigments from the resin systems, reducing the hiding power and aesthetics of the final coatings.
The interactions between pigments and other paint/ink ingredients are complex, and extensive efforts are required to understand these interactions and how these interactions affect the properties and performances of the final products. For these purposes, dispersants have been extensively used in coating formulations to help the dispersion of pigments and extend shelf life of the resulting paints/inks. Numerous pigment pre-treatment methods have been utilized to increase the pigment compatibility in the resin system and to improve pigment dispersion, control system viscosity, and maintain stability of the paint/ink systems. The surface of the pigment can be treated with surface active reagents that can be either physically absorbed onto the pigment surfaces or chemically bonded to the pigment surfaces. Examples of such surface active reagents include fatty acids, organophosphorous compounds, and silane coupling reagents. Polymeric materials that contain surface active groups also have been extensively studied as pigment dispersants. Research also has been conducted to encapsulate pigments into polymer or inorganic matrixes to improve their dispersion in paint/ink systems.
Pigments can react and/or interact with other ingredients of the paints/inks. In these cases, to increase storage stability and to enhance the final performance of paints/inks, it is desirable to separate these ingredients from one another and mix them together only immediately prior to the application of the paints/inks. However, due to the complex nature of these coating materials, commercial paints/inks are generally supplied as finished products with all ingredients and processed to their final application form by their manufactures. Therefore, despite extensive efforts to mitigate the undesirable reactions and interactions among different ingredients, commercial paints/inks generally have limited shelf-lives. For example, aluminum pigments, which are widely used in paint/ink industries to generate metallic effects, can readily react with water and liberate hydrogen gas. Not only will the erosion of aluminum reduce the metallic luster of the final coating, excessive hydrogen buildup inside the container will impose great hazards during storage, shipping, and handling. With current industrial focus shifts from solvent-borne to water-borne to minimize volatile organic contents (VOC), extensive industrial efforts have been diverted to mitigate the reaction between aluminum pigments and water and to elongate the shelf-stability of the final products.
The interaction between pigment and other coating components also affect the final coating properties such as adhesion, weathering, chemical resistance, and aesthetics. For example, pigments tend to migrate to the surface of coating if they are incompatible with the resin matrix. In these cases, pigments have less adhesion to the film and are exposed to the external environments. As a result, the final coating will have limited scratch resistance and be prone to discoloration.
Therefore, due to the complex interactions between pigments and the rest of the paint/ink systems, extensive considerations have to be made during the paint/ink formulation processes. For instance, depending on the compatibility of the pigments and the rest of the paint/ink systems, extensive research would be needed to identify suitable additives and to develop proper processes to achieve the desirable properties of the paint/inks.